1. Field of the Invention
The present invention relates to a method of performing secondary modulation on a train of data acquired by run length limited modulation of a train of data that represents information to be recorded on a recording medium.
2. Description of the Related Art
Run length limited (hereinafter referred to as "RLL") modulation is one type of modulation to convert a train of data representing information to be recorded on a recording medium, to have a format suitable for the recording medium. A series of zeros or ones in a train of NRZ (Non Return to Zero) data is called "run", and modulation to encode input data according to conversion rules is called (d, k) RLL modulation where d is the minimum run of zeros and k is the maximum run of zeros. In (1, 7) RLL modulation, one type of a RLL modulation, NRZ data is converted in accordance with Tables A and B given in FIG. 1. If the NRZ data in the data input order matches with any 4-bit pattern in Table B, the four bits of the NRZ data are converted into six bits in accordance with the Table B. If the NRZ data in the data input order matches with none of the 4-bit patterns in Table B, every two bits of the NRZ data are converted into three bits in accordance with the Table A. When the result of the (1, 7) RLL modulation is further subject to NRZI (Non Return to Zero Inverted) conversion, a series of ones have seven NRZI patterns 2T to 8T where T represents a bit interval.
In a recording medium, such as a magneto optical disk or a write once optical disk, on which recording marks (or pits) are formed on the recording surface by a temperature rise caused by a focused laser beam at the time of information recording, if the mark length becomes long even with constant write power of the laser beam, heat energy is accumulated in the recording medium so that the mark width gradually increases and a mark edge shift occurs. For example, if five consecutive bits in a data train are all ones, when constant write power is generated as shown in FIG. 2(a), a mark is formed on the recording medium as shown in FIG. 2(b). The position of the mark which corresponds to the trailing edge of the data where "1" changes to "0" in FIG. 2(a) is shifted, producing a mark edge shift A. When a signal is read out from a portion of the recording medium having such a mark, the read signal would have a waveform as shown in FIG. 2(c) so that the waveform above the slice level contains the mark edge shift A, preventing the reproduction of the original data.
Heretofore, to overcome such a shortcoming, write power at the time of irradiating a laser beam has been controlled in order to form marks of a given width without any mark edge shift. Alternatively, when (1, 7) RLL modulation and NRZI conversion are employed as the first modulation as described above, the NRZI patterns 2T to 8T are converted by the secondary modulation as shown in FIG. 3 to write a signal resulting from the secondary modulation on the recording medium.
However, to control the write power at the time of irradiating a laser beam requires different power controls for the different patterns, making the structure of the power controller very complicated. If the above secondary modulation is carried out, when "11111" is converted to "10010", for example, for an odd-numbered pattern of 5T or more, the level of a read signal corresponding to the pattern portion in the recording area at the outer periphery side of the disk where the recording density gets lower drops below the slice level as shown in FIG. 4, the signal cannot be demodulated to "11111" at the individual bit discrimination positions (points of the arrows).